Gravitts are a species native to SR388 in Metroid II: Return of Samus. This creature hides in the floor, only coming out to attack when Samus comes close. When hidden, only the spike on top can be seen, which cannot be shot with a narrow-beamed weapon. Samus must jump over and shoot down at them while they are hidden. These creatures are notorious for hiding under destructible Sand blocks, so caution should be exercised when traveling on the bottom of a sandy area.
In Samus Returns, a black variant is encountered that is invulnerable to most weapon fire but is susceptible to the Beam Burst and Power Bombs.
Metroid II manual
"It comes out of the ground and rushes at Samus, then darts back underground."
"Gravitts look an awful lot like moving rocks; the way they bury themselves until only their tips are visible enhances the effect. Their rocky appearance isn’t just for show; they have strong outer shells but move slowly when compared to most other creatures in the planet’s underground. Defeating them requires explosive weaponry."
Walkthrough (p. 36)
"These creatures hide underground and don’t pop up unless disturbed. You can see the tips of their rocky shells, which can be shot from a distance to force them out from under the earth. They can’t be easily defeated with the Power Beam, but a few well-placed missiles make short work of them. If you allow them to get close, they lunge at you. The lunge isn’t particularly fast and is also counterable. All in all, these foes only cause a problem when other enemies are around."
Walkthrough (p. 103)
"This creature packs a hardy shell on its back that’s impervious to most attacks. Black Gravitts aren’t particularly difficult to defeat, and the greatest threat they pose comes when they pop out of the ground. Turn on you Beam Burst and let ’em have it. They’ll be out of your hair in no time flat."